import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-3, 3, 50)
y = 2 * x + 1

plt.figure(num=1, figsize=(8, 5),)
plt.plot(x, y)

ax = plt.gca()
ax.spines['right'].set_color('none')
ax.spines['top'].set_color('none')

ax.xaxis.set_ticks_position('bottom')
ax.yaxis.set_ticks_position('left')

ax.spines['bottom'].set_position(('data', 0))
ax.spines['left'].set_position(('data', 0))

x0 = 1
y0 = 2 * x0 + 1

plt.scatter(x0, y0, s=50, color='blue')
plt.plot([x0, x0], [y0, 0], 'k--', linewidth=2.5) # k是黑色，--表示虚线

# method 1
##########
plt.annotate(r'$2x+1=%s$' % y0, xy=(x0, y0), xycoords='data', xytext=(+30, -30), textcoords='offset points', fontsize=16, arrowprops=dict(arrowstyle='->', connectionstyle='arc3, rad=.2'))

# method 2
##########
plt.text(-3.7, 3, r'$This\ is\ the\ some\ text.\mu\ \sigma_i\ \alpha_t$', fontdict={"size": 16, "color": 'red'})
plt.show()

